1. Field of the Invention
The present invention relates to a liquid crystal display panel and more particularly to a liquid crystal display panel having an applied horizontal electric field that is capable of improving viewing angle and brightness.
2. Description of the Related Art
A liquid crystal display displays pictures by adjusting the light transmittance of a liquid crystals using an electric field. Liquid crystal displays are classified into a vertical electric field type and a horizontal electric field type in accordance with a direction of the electric field driving the liquid crystal.
The liquid crystal display having an applied horizontal electric field in which a common electrode formed on an upper substrate and a pixel electrode formed on a lower substrate are arranged to face each other, drives a liquid crystal of a twisted nematic mode (TN) by a vertical electric field formed between the common electrode and the pixel electrode. The liquid crystal display having the applied horizontal electric field has an advantage of a large aperture ratio, while it has a defect of a narrow viewing angle of about 90°.
The liquid crystal display having the applied horizontal electric field type drives a liquid crystal of an in plane switch (hereinafter) mode by applying a horizontal electric field between the pixel electrode and the common electrode disposed in parallel on the lower substrate. The liquid crystal display using the horizontal electric field has an advantage of a wide viewing angle of about 160°. Hereinafter, the liquid crystal display using the horizontal electric field is explained in full detail.
The liquid crystal display using the horizontal electric field includes a thin film transistor array substrate (a lower substrate) and a color filter array substrate (an upper substrate) that face and join each other, a spacer for uniformly maintaining a cell gap between the two substrates and liquid crystal injected into a space provided by the spacer.
The thin film transistor array substrate includes a plurality of signal lines for forming a horizontal electric field by a pixel, a plurality of thin film transistors, and an alignment film for aligning the liquid crystal. The color filter array substrate includes a color filter for producing colors, a black matrix for preventing a light leakage and an alignment film applied for a liquid crystal alignment thereon.
A multi domain liquid crystal display panel has been proposed for aligning the liquid crystals in each pixel in a different direction from each other in order to compensate for a narrow viewing angle of the liquid crystal display.
FIG. 1 is a plane view schematically illustrating a portion of the thin film transistor array substrate of the multi domain liquid crystal display panel using the horizontal electric field.
The thin film transistor array substrate shown in FIG. 1 comprises gate lines 2 and data lines (not shown) formed substantially perpendicular to one another on a lower substrate, a thin film transistor (not shown) formed for each interconnection part, pixel electrodes 14 and common electrodes 18 formed in order to apply the horizontal electric field in a pixel region 5 defined by the interconnection part and common lines 16 connected to common electrodes 18.
The gate line 2 supplies a gate signal to the gate electrode (not shown) of the thin film transistor. The data line supplies a pixel signal to the pixel electrode 14 via the drain electrode (not shown) of the thin film transistor. The gate line 2 and the data line 4 are formed in an interconnection structure to thereby define the pixel region 5.
The common line 16 formed in parallel with the gate line 2 with the pixel region 5 positioned between the common line 16 and the gate line 2 to supply a reference voltage for driving the liquid crystal to the common electrode 18.
The thin film transistor responds to the gate signal of the gate line 2 so that the pixel signal of the data line may be charged to the pixel electrode 14. To this end, the thin film transistor includes a gate electrode connected to the gate line 2, a source electrode (not shown) connected to the data line and a drain electrode connected to the pixel electrode 14.
The pixel electrode 14, which is connected to the drain electrode 12, comprises a horizontal part 14A formed in parallel with adjacent gate line 2 and a finger part 14B connected to the horizontal part 14A and formed in parallel with the common electrode 18. The finger part 14B of the pixel electrode is formed symmetrically with the pixel electrode 14 adjacent in association with the gate line 2 and is formed in parallel with the pixel electrode 14 adjacent with reference to the data line 4.
The common electrode 18 is connected to the common line 16 and is formed in the pixel region 5. Specifically, the common electrode 18 is formed in parallel with the fingerpart 14B of the pixel electrode 14 in the pixel region 5.
Accordingly, a horizontal electric field is formed between the pixel electrode 14 to which the pixel signal is supplied via the thin film transistor and the common electrode 18 to which the reference voltage is supplied via the common line 16. Specifically, the horizontal electric field is formed between the finger part 14B of the pixel electrode 14 and the common electrode 18. The liquid crystal molecules 13 arranged in the horizontal direction between the thin film transistor array substrate and the color filter array substrate by the horizontal electric field rotate due to a dielectric anisotropy. The light transmittance transmitting the pixel region 5 differs in accordance with the amount of rotation of the liquid crystal molecules. Thus, the pictures can be represented.
In FIG. 2, in one aspect, the pixels are comprised of sub-pixels of R, G and B. The alignment direction of the liquid crystal molecules for each of the gate lines 2 is opposite to each other in order to improve the viewing angle. That is, the common electrode 18 and the pixel electrodes 14 of a first sub-pixel (A) connected to Kth gate line (GK) are tilted by a predetermined angle from the gate line 2 to a first direction (S1) and the common electrode 18 and the pixel electrodes 14 of a second sub-pixel (B) connected to Kth+1 gate line (GK+1) are tilted by a predetermined angle from the gate line 2 to a second direction (S2).
Accordingly, the direction of the liquid crystal molecules 13 located in the first sub-pixel (A) is opposite from an alignment direction of the liquid crystal 15 of the second sub-pixel (B).
The liquid crystal display of the horizontal electric field applying type according to the related art, as shown in FIG. 2, has an advantage that the viewing angle is excellent because of a 2-domain structure where the liquid crystal alignment direction is symmetrically aligned with reference to the gate line 2.
On the other hand, with respect to the liquid crystal display using vertical electric field and using horizontal electric field, one pixel is comprised of R, G and B sub-pixels. In each of the R, G and B sub-pixels, if a light quantity provided from a back light unit is 100%, a light quantity provided from the upper substrate via the color filter is about 27˜33%. Thus, the brightness of the liquid crystal display panel of which each pixel is comprised of R, G and B sub-pixels is lowered respectively.
To solve this problem, as shown in FIG. 3, a liquid crystal display having a structure where each of the pixels is comprised of the R, G, B and W sub-pixels has been proposed. With respect to the W sub-pixel of the liquid crystal display panel of the quad structure, if the light quantity provided from the back light unit is 100%, the light quantity provided from the upper substrate via a transparent color filter is more than about 95%. Accordingly, since the brightness lowered due to R, G and B sub-pixels may be compensated, the brightness of the liquid crystal display panel comprised of R, G, B and W sub-pixels may be improved as compared with the liquid crystal display panel comprised of R G and B sub-pixels.
Further, the number of driver ICs may be reduced by supplying two data signals (RW and GB) to one data line. With respect to each sub-pixel, the alignment directions of the liquid crystal molecules with reference to the gate line 52 are formed opposite to each other for compensating the viewing angle. That is, as shown in FIG. 3, the alignment directions of the liquid crystal molecules of each pixel of R sub-pixel and W sub-pixel and G sub-pixel and B sub-pixel are opposite each other.
However, in the R, G, B, W structure holding in common two gate lines 52, since the liquid crystal alignment direction of each sub-pixel of ith pixel is identical to that of each sub-pixel of (i+2) th pixel and the alignment direction of each sub-pixel of (i+1)th pixel is identical to that of each sub-pixel of (i+3)th pixel with reference to the gate line 52 respectively, it is difficult to compensate the viewing angle.